IEEE Electrification Magazine - September 2014 - 14

(A)

ia

ib ic

150
100
50
0
-50
-100
-150
0.8

1

1.2

1.4

1.6

UF%

Time (s)
(a)
3
2
1
0

Substation
Without Load

Load On
Compensation Off

0.8

1

Load On
Compensation On

1.2

1.4

1.5%

1.6

Time (s)
(b)

Figure 18. (a) The line currents and (b) the voltage UF%.

and capacitors of the three studied topologies are evaluated and used in
1
E cap = 2 CV| 2

1
E ind = 2 LI| 2 .

(8)

Figure 16 shows a comparison of the total energies for
the three solutions. Comparing the energy stored in the
reactive elements for the three topologies, a huge
difference exists between the proposed compensator and
the classical solutions based on VSI
converters. Particularly for the size of
the dc-link capacitors, the capacitive
stored energy in these conversion
structures is significant. In fact, as
the converter is injecting a purely
negative sequence three-phase current, the fluctuating power makes it
necessary to install large capacitors
to limit the voltage ripple at the
dc side.

Simulation Results of the
Chopper-Controlled Steinmetz
Circuit

The semiconductor
power losses and
energy-storage
requirements
compared to the
widely used VSI
topology make the
proposed solution
very attractive for
railway operators.

The worst-case condition, i.e.,
at lowest short-circuit power,
S CC = 295 MVA, is considered. The
chopper-controlled Steinmetz circuit
is connected in parallel to the substation. In the circuit, the substation
and the trains were replaced by a controlled current
source. Then, simulations with PSIM software were carried out using measured current waveforms. The substation current waveform is given in Figure 17 and presents

14

I E E E E l e c t r i f i c ati o n M agaz ine / september 2014

a third harmonic of about 20  A. Resulting line currents
and UF% are presented in Figure 18, in which three
working periods can be distinguished in the simulation
corresponding to three modes of operation:
xx
The substation is not loaded and appears as a balanced load to the power network.
xx
The substation is loaded, and the UF reaches 2%.
xx
The chopper-controlled Steinmetz circuit is turned on
and UF is close to zero, well under the limit of 1.5%.
Figure 19 shows a zoom on the
three-phase line-currents and the
currents drawn by the compensator. It
can be seen that currents i ca and i cb
are quasi-sinusoidal, which confirms
that harmonic interactions are avoided, as expected, with the frequency
analysis presented above. Furthermore, the line voltage drop corresponding to the negative current
sequence is strongly reduced, and the
substation voltage is boosted by 1.7 %.

Conclusion

In this article, reactive power and
voltage unbalance compensators
based on PWM ac choppers were proposed. In multilevel structures, current or voltage sharing is naturally
ensured by the choice of impedance
values. A very simple control of reactive power can be achieved by varying only the duty cycle;
no control loops for internal variables are required. Compared to a TCR solution, the ac chopper does not generate
any low-order harmonics, thanks to its PWM operation.



Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014

IEEE Electrification Magazine - September 2014 - Cover1
IEEE Electrification Magazine - September 2014 - Cover2
IEEE Electrification Magazine - September 2014 - 1
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IEEE Electrification Magazine - September 2014 - Cover3
IEEE Electrification Magazine - September 2014 - Cover4
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https://www.nxtbook.com/nxtbooks/pes/electrification_march2022
https://www.nxtbook.com/nxtbooks/pes/electrification_december2021
https://www.nxtbook.com/nxtbooks/pes/electrification_september2021
https://www.nxtbook.com/nxtbooks/pes/electrification_june2021
https://www.nxtbook.com/nxtbooks/pes/electrification_march2021
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https://www.nxtbook.com/nxtbooks/pes/electrification_march2020
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https://www.nxtbook.com/nxtbooks/pes/electrification_september2019
https://www.nxtbook.com/nxtbooks/pes/electrification_june2019
https://www.nxtbook.com/nxtbooks/pes/electrification_march2019
https://www.nxtbook.com/nxtbooks/pes/electrification_december2018
https://www.nxtbook.com/nxtbooks/pes/electrification_september2018
https://www.nxtbook.com/nxtbooks/pes/electrification_june2018
https://www.nxtbook.com/nxtbooks/pes/electrification_december2017
https://www.nxtbook.com/nxtbooks/pes/electrification_september2017
https://www.nxtbook.com/nxtbooks/pes/electrification_march2018
https://www.nxtbook.com/nxtbooks/pes/electrification_june2017
https://www.nxtbook.com/nxtbooks/pes/electrification_march2017
https://www.nxtbook.com/nxtbooks/pes/electrification_june2016
https://www.nxtbook.com/nxtbooks/pes/electrification_december2016
https://www.nxtbook.com/nxtbooks/pes/electrification_september2016
https://www.nxtbook.com/nxtbooks/pes/electrification_december2015
https://www.nxtbook.com/nxtbooks/pes/electrification_march2016
https://www.nxtbook.com/nxtbooks/pes/electrification_march2015
https://www.nxtbook.com/nxtbooks/pes/electrification_june2015
https://www.nxtbook.com/nxtbooks/pes/electrification_september2015
https://www.nxtbook.com/nxtbooks/pes/electrification_march2014
https://www.nxtbook.com/nxtbooks/pes/electrification_june2014
https://www.nxtbook.com/nxtbooks/pes/electrification_september2014
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